1. Field of the Invention
The present invention relates generally to a capacitor unit included in an electrolytic capacitor used in a high-speed power source circuit, and to a method for producing the same.
2. Related Background Art
Conventionally, an electrolytic capacitor in which a valve metal such as aluminum or tantalum is used, and a multilayer ceramic capacitor in which Ag/Pd or Ni is used for electrodes and barium titanate is used as a dielectric, have been known as capacitors. These capacitors are used in most power source circuits. Recently, since CPU driving circuits and switching power source circuits particularly are required to be driven with a lower driving voltage, to consume less power, and to be suitable for high frequencies, a capacitor also is required to have a large capacitance, a low equivalent series resistance (hereinafter referred to as ESR), and a low equivalent series inductance (hereinafter referred to as ESL). To meet with these requirements, particularly for a capacitor to have a low ESR, a technique in which a specialty polymer having a high electroconductivity (conductive polymer) is used as the solid electrolyte for the cathode has been examined and developed.
A configuration of a conventional specialty polymer aluminum electrolytic capacitor is described below, with reference to FIG. 5. FIG. 5 is a cross-sectional view of a conventional specialty polymer aluminum electrolytic capacitor. In FIG. 5, 101 denotes an aluminum foil for an anode (hereinafter referred to as anode-use aluminum foil). 102 denotes a dielectric layer. 103 denotes a conductive polymer layer. 104 denotes a carbon layer. 105 denotes an Ag paste layer. 106 denotes an anode terminal. 107 denotes a cathode terminal. 108 denotes a molding resin.
The anode-use aluminum foil 101 has been treated so as to have rough surfaces, and is provided with the dielectric layer 102 on the surfaces. On the surfaces of the anode-use aluminum foil 101 provided with the dielectric layer 102, the conductive polymer layer 103 made of polypyrrole, polythiophene, polyaniline, etc. is formed. Furthermore, on the conductive polymer layer 103, the carbon layer 104 and the Ag paste layer 105 are formed in the stated order, so that a conventional capacitor unit is provided. Further, the anode terminal 106 and the cathode terminal 107 are bonded with the foregoing conventional capacitor unit, and the capacitor unit is sealed with the molding resin 108. Thus, the conventional specialty polymer aluminum electrolytic capacitor is formed.
Such a conventional specialty polymer aluminum electrolytic capacitor has a lower ESR than that of an electrolytic capacitor using an electrolytic solution as electrolyte (hereinafter referred to as an electrolytic solution-type electrolytic capacitor). To further increase the capacitance and decrease the ESR, however, a configuration in which a plurality of conventional capacitor units are laminated with an Ag adhesive (Ag adhesive paste) has been developed. Furthermore, as to the foregoing conventional capacitor unit, to decrease the ESR thereof further, materials for the conductive polymer layer 103, the carbon layer 104, and the Ag paste layer 105 have been developed.
Furthermore, to decrease the ESR further, conventionally, another capacitor unit has been proposed in which a cathode-use aluminum foil on which a dielectric layer, and a metallic element that functions as a charge collector for the cathode (hereinafter referred to as cathode-use charge collecting metallic element) are bonded with each other with only a conductive polymer layer provided therebetween (JP11(1999)-219861A).
However, a specialty polymer aluminum electrolytic capacitor having the above-described conventional capacitor unit undergoes drawbacks as described below.
First of all, it is difficult to decrease the ESR of the conventional specialty polymer aluminum electrolytic capacitor to as low a level as that of the multilayer ceramic capacitor, and it cannot be achieved sufficiently by decreasing specific resistances of the conductive polymer layer, the carbon layer, and the Ag paste layer. This is evident from the fact that, in the case where an electrolytic capacitor using a specialty polymer is compared with an electrolytic solution-type electrolytic capacitor, the specialty polymer has a degree of conductivity that is two or more digits greater than that of the electrolytic solution, but the electrolytic capacitor using the specialty polymer has an ESR that is only one digit or the like smaller than that of the electrolytic-solution-type electrolytic capacitor. In other words, it is necessary to develop a technique other than that for lowering of the specific resistances of the materials, that is, to develop a technique for reducing various interface resistances at an interface between a conductive polymer layer and a carbon layer, at an interface between a carbon layer and an Ag paste layer, and the like to cause an electrolytic capacitor using a specialty polymer to have an ESR that is as low as that of a multilayer ceramic capacitor.
On the other hand, in the case where an anode-use aluminum foil and a cathode-use charge collecting metallic element are bonded with each other via a conductive polymer layer, interfaces are fewer in number as compared with the foregoing case, and hence it is possible to obtain a further lower ESR. In this case as well, however, a drawback arises in that a producing process is complex, since the producing process requires the following step: a step of assembling an anode-use aluminum foil on which a dielectric layer is formed and a cathode-use charge collecting metallic element so that they are facing each other, and filling a space between the anode-use aluminum foil and the cathode-use charge collecting metallic element with a conductive polymer for bonding use so that a layer of the conductive polymer is obtained in the space; or a step of assembling the cathode-use charge collecting metallic element and an anode-use aluminum electrolytic foil in the vicinity so that they are facing each other, and placing the same in an electrolytic solution so that a conductive polymer layer is formed from the cathode-use charge collecting metallic element side by electrolytic polymerization.
It is an object of the present invention to provide a capacitor unit in which respective interface resistances at interfaces between layers are decreased without complex processes so as to obtain as low an ESR as that of a multilayer ceramic capacitor, a method for producing the same, and an electrolytic capacitor in which the same is used.
To achieve the foregoing object, a capacitor unit of the present invention includes a valve metal element for an anode that has a rough surface, a dielectric layer provided on the surface of the valve metal element for the anode, a conductive layer for a cathode, provided on the dielectric layer, and a charge collecting metal element for the cathode provided on the conductive layer for the cathode, wherein the conductive layer for the cathode includes three layers that are a first conductive polymer layer, a second conductive polymer layer, and a conductive adhesive layer interposed between the first and second conductive polymer layers.
Thus, in the capacitor unit of the present invention, the conductive layer for cathode has a three-layer structure. Besides, the first conductive polymer layer and the second conductive polymer layer are provided on the valve metal element for the anode and the charge collecting metal element for the cathode, respectively, and the first and second conductive polymer layers are made to adhere to each other with the conductive adhesive layer. Therefore, upon integrally providing the components into a capacitor unit, the first conductive polymer layer and the second conductive polymer layer may be provided beforehand on the valve metal element for the anode and the charge collecting metal element for the cathode, respectively, and laminated on each other with the conductive adhesive layer interposed therebetween, while being compressed in the lamination direction. By thus compressing the same, contact areas between layers, for instance between the charge collecting metal element and the second conductive polymer layer, are increased. Therefore, various interface resistances, for instance, an interface resistance between the charge collecting metal element for the cathode and the second conductive polymer layer, can be decreased.
Furthermore, the first and second conductive polymer layers are made of relatively soft materials. Therefore, in the configuration in which the conductive adhesive layer is interposed between the first and second conductive polymer layers, the contact areas between the three layers are large, and interface resistances are suppressed to a low level. Therefore, a resistance in the conductive layer for the cathode composed of the foregoing three layers also is suppressed to a low level. Furthermore, with sufficient compression upon integration into the capacitor unit, the resistance in the conductive layer for cathode can be suppressed to a still lower level.
Furthermore, by providing the conductive adhesive layer, it is possible to prevent the first and second conductive polymer layers from being separated from each other, and hence, to maintain the same stably with increased contact areas.
As described above, the configuration of the present invention ensures reduction of interface resistances between the respective layers, thereby achieving decreased ESR.
Furthermore, in the capacitor unit of the present invention, the conductive adhesive layer preferably is made of a thermosetting resin in which conductive particles are dispersed.
Furthermore, in the capacitor unit of the present invention, the valve metal element for the anode preferably is made of one of an aluminum foil, a tantalum foil, and a niobium foil.
Furthermore, in the capacitor unit of the present invention, the cathode charge collecting metal element for the cathode preferably is made of a metal foil in which carbon particles are embedded so as to be exposed on the surface of the metal foil. This configuration suppresses an interface resistance between the charge collecting metal element for the cathode and the second conductive polymer layer, thereby allowing the ESR to be lowered further.
Furthermore, a method for producing the capacitor unit of the present invention includes the steps of: forming the first conductive polymer layer on the dielectric layer formed on the surface of the valve metal element for the anode; forming the second conductive polymer layer on a surface of the charge collecting meal element for the cathode; applying a conductive adhesive over at least one of the first conductive polymer layer and the second conductive polymer layer; and laminating the valve metal element for the anode and the charge collecting metal element for the cathode so that the first conductive polymer layer and the second conductive polymer layer face each other via the conductive adhesive, and curing the conductive adhesive in a state of being compressed in a lamination direction.
This method allows interface resistances between the layers to decrease, thereby decreasing the ESR, without complex processes. It should be noted that effects of the compression are as described above.
To achieve the aforementioned object, an electrolytic capacitor of the present invention includes the aforementioned capacitor unit, a molding material for molding the capacitor unit, an anode terminal connected with the valve metal element for the anode, and a cathode terminal connected with the charge collecting metal element for the cathode.
Since the electrolytic capacitor of the present invention includes the capacitor unit of the present invention with which the ESR can be lowered, in the electrolytic capacitor, it is possible to obtain as low an ESR as that of a multilayer ceramic capacitor.
Furthermore, the electrolytic capacitor of the present invention may include a plurality of the capacitor units that are laminated, thereby constituting a capacitor unit lamination.
Furthermore, in the electrolytic capacitor of the present invention, elastic bodies preferably are provided between the upper and lower surfaces of either the capacitor unit or the capacitor unit lamination on one hand, and the molding material on the other hand. With this configuration, even after packaging, because of the elastic bodies, a compressing force is exerted on the capacitor unit or the capacitor unit lamination in the lamination direction. Therefore, stable interface connections are achieved.
These and other objects, characteristics, and advantages will be fully revealed by the following description. The benefits of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.